**Al-Cr-Y-N**

The oxidation behaviour of Al27Cr46N50 and Y-doped coatings was investigated. Al26.5Cr22.5Y1N50 and Al26.5Cr21.5Y2N50 coatings showed improved oxidation resistance, whereas a further increase (4 at.% in the coating) in Y had a negative influence. Experimental and computational studies on the effect of yttrium on the phase formation of sputtered (AlCrY)N showed a decrease in the maximum Al content with retained fcc structure to 68 at.% at a Y content of 2 at.% of the total metal content. The authors concluded that, theoretically, the Al34Cr15Y1N50 coatings exhibit the most promising oxidation resistance within the group of coatings containing Y as compared to (AlCr)N coatings [109,110]. (CrAlY)N coatings with a Y content up to 2.3 at.% were co-sputtered using Cr50Al50-composite and pure Y targets. The hardness increased from ca. 16 to 24 GPa with increasing Y content. However, oxidation experiments (1100 ◦C) demonstrated a lower Y content of 0.3 to 0.7 at.% (e.g., Al23.8Cr23.1Y0.7N50O2.4) to be beneficial for the oxidation resistance. In excess of 1.3 at.% Y, the oxidation resistance deteriorated as the result of the formation of porous and non-protective oxide scales [111]. A further study of co-sputtered coatings containing Y showed that excellent oxidation behaviour was achieved with a Y content of 3.4 at.% for Cr-rich Cr25.8Al15.3Y3.4N55.5 coatings, whereas for Al-rich coatings, a lower Y content of 2.6 at.% was the best with Al24.9Cr18.1Y2.6N54.4. It was also found that higher Y contents promote the hcp phase formation [112].

It can thus be concluded that a low Y content in the range of about 0.7 to 3.4 at.% of the total elemental composition of the coating can have a positive effect on the oxidation. However, the absolute value seems to be dependent on the Al/Cr ratio in the coating.

#### **Al-Cr-La-N**

Al29.1Cr13.5N57.4 and (AlCrLa)N coatings were deposited by sputtering using Al70Cr30 and La targets. The concentration of La was varied in the range of 1.39 to 7.73 at.% by adjusting the sputtering power for the La target. A small amount of La, e.g., Al28.1Cr12.5La1.4N58, formed a solid solution in the (AlCr)N lattice, resulting in grain refinement. The hardness of (AlCrLa)N containing 1.4 at.% La was significantly higher than that of the undoped

(AlCr)N. This coating showed the lowest wear rate. An increased amount of La resulted in a grain boundary segregation, thus an nc-(AlCrLa)N coating consisting of an (AlCrLa)N matrix combined with a-La2O3/a-LaN segregations. The hardness decreased with an increase in La content at the same time as the friction value (against Si3N4) was reduced [113].
